Method And Apparatus For Obtaining A Compressed Nitrogen Product

ABSTRACT

A method and apparatus to obtain a compressed nitrogen product by low-temperature fractionation of air in a distillation column system. The system has a high-pressure column, a low-pressure column, a main condenser, and a low-pressure column top condenser. Bottoms liquid from the low-pressure column is evaporated in the top condenser and the gas formed is decompressed to perform work that drives a cold compressor. A gaseous first compressed nitrogen product stream from the high-pressure column is warmed in the main heat exchanger. A further gaseous nitrogen stream from the low-pressure column is compressed in the cold compressor and warmed as a second compressed nitrogen product stream in the main heat exchanger. The cold compressor overcomes a pressure differential which is at least equal to two thirds of the pressure differential between the top of the high-pressure column and the top of the low-pressure column.

The invention related to a method according to the preamble of patent claim 1.

The principles of low-temperature fractionation of air generally and the structure of two-column plants specifically are described in the monograph “Tieftemperaturtechnik” by Hausen/Linde (2nd Edition, 1985) and in an article by Latimer in Chemical Engineering Progress (Vol. 63, No. 2, 1967, page 35). The heat-exchanging relationship between the high-pressure column and the low-pressure column of a double column is generally realized by way of a main condenser, in which top gas of the high-pressure column is liquefied against evaporating bottoms liquid of the low-pressure column.

In the invention, the main condenser and the low-pressure-column top condenser take the form of condenser-evaporators. The expression “condenser-evaporator” refers to a heat exchanger in which a first, condensing fluid stream enters into indirect heat exchange with a second, evaporating fluid stream. Each condenser-evaporator has a liquefaction space and an evaporation space, which consist of liquefaction passages and, respectively, evaporation passages. The condensation (liquefaction) of the first fluid stream takes place in the liquefaction space, the evaporation of the second fluid stream in the evaporation space. The evaporation space and the liquefaction space are formed by groups of passages which are in a heat-exchanging interrelationship.

Here, each condenser can be formed by a single heat-exchanger block or else by a plurality of heat-exchanger blocks which are arranged in a common pressure vessel. Both can be designed as single- or multi-level bath evaporators, forced-flow evaporators or else as falling-film evaporators. Additionally, the main condenser can be designed as a cascade evaporator, for example as described in EP 1287302 B1=U.S. Pat. No. 6,748,763 B2.

A “main heat exchanger” serves for cooling feed air in indirect heat exchange with return streams from the distillation column system. Said heat exchanger can be formed from a single heat exchanger section or a plurality of parallel- and/or series-connected heat exchanger sections, for example from one or more plate heat exchanger blocks.

A method of the type specified at the outset is known from U.S. Pat. No. 4,453,957. Here, the mechanical energy produced in the tail-gas turbine is used solely for refrigeration.

The object of the invention is to specify a method and a corresponding apparatus with which it is possible to obtain the nitrogen stream from the low-pressure column at at least high-pressure-column pressure, and at the same time to have an especially low energy consumption.

This object is achieved by the features of claim 1.

Here, a cold compressor, which is driven through the use, at least in part, of the mechanical energy produced in the first tail gas turbine (33), is directly used here to compress nitrogen product, namely that of the low-pressure column which is brought, for example, to approximately high-pressure-column pressure or to a higher pressure.

In the context of the invention, it has surprisingly been found that, with this method, the method according to the invention brings the low-pressure-column nitrogen up to the pressure level of the high-pressure-column nitrogen and is therefore favorable in terms of energy, As an additional effect, the result is a relatively simple method and thus also a relatively low apparatus complexity, in particular for the main heat exchanger.

In the method according to the invention, the following pressure ranges are used:

-   -   High-pressure column (at the top): for example 12 to 17 bar,         preferably 13 to 16 bar     -   Low-pressure column (at the top): for example 6 to 10 bar,         preferably 7 to 9 bar

In the invention, it is possible in principle for the first compressed nitrogen product stream and the second compressed nitrogen product stream to be warmed separately in the main heat exchanger. Preferably, however, the first compressed nitrogen product stream and the second compressed nitrogen product stream are mixed upstream of the main heat exchanger.

If required, an additional, third compressed nitrogen stream can be formed by another part of the nitrogen product of the low-pressure column, in that said part is guided directly into the main heat exchanger and is discharged as product at the low-pressure-column pressure (minus pressure losses).

In a first variant of the energy transfer between the first tail-gas turbine and the cold compressor, the first tail-gas turbine and the cold compressor are mechanically coupled. This can be accomplished by way of a common shaft or a gear mechanism.

For the generation of refrigeration, it is possible for the tail-gas turbine to be mechanically coupled to a generator or to an oil brake.

In a second variant of the energy transfer between the first tail-gas turbine and the cold compressor, the first tail-gas turbine is mechanically coupled to an electrical generator and the cold compressor is driven by an electric motor; the energy produced in the generator is then electrically transferred to the motor and thus drives the cold compressor.

Alternatively, a second part of the tail gas warmed to the intermediate temperature can be expanded in a work-performing manner in a second tail-gas turbine which is connected in parallel with the first tail-gas turbine which is coupled to the cold compressor. It is then possible for the first tail-gas turbine to be coupled only to the cold compressor, and for the second tail gas turbine to be coupled to a generator or to a dissipative brake.

If the high-pressure column is insufficient, the first, the second or both. compressed nitrogen streams can be further compressed downstream of the main heat exchanger in a nitrogen compressor. Preferably, both compressed nitrogen streams are brought to a higher pressure together in the nitrogen compressor.

In this case, it is favorable to combine the air compression and the nitrogen compression in a single machine, in that the feed air is compressed in a main air compressor which is formed by the first i stages of a combined n-stage compressor, where n≧2, i<n. At the same time, the nitrogen compressor is formed by the n−i last stages of the combined n-stage compressor. For example, an eight-stage compressor is used, the three to four last stages of which are used as the nitrogen compressor.

The invention also relates to an apparatus for obtaining a compressed nitrogen product by low-temperature fractionation of air as claimed in patent claim 14.

The apparatus according to the invention can be supplemented by one, more than one or all features of the independent method claims.

The invention and further details of the invention are explained in more detail below on the basis of exemplary embodiments represented in the drawings, in which:

FIG. 1 shows a first exemplary embodiment of the invention with a single tail-gas turbine,

FIG. 2 shows a second exemplary embodiment with two tail-gas turbines,

FIG. 3 shows a modification to FIG. 1 with a combined compressor,

FIG. 4 shows a further modification to FIG. 1 with separate warming of the two compressed nitrogen product streams,

FIG. 5 shows an embodiment with electrical energy transfer between the first tail-gas turbine and the cold compressor,

FIG. 6 shows a modification with the product pressure slightly below the high-pressure column pressure,

FIG. 7 shows an exemplary embodiment similar to FIG. 6, but with forced-flow evaporators, and

FIG. 8 shows a system similar to that in FIG. 4, but with columns arranged. next to one another.

In FIG. 1, atmospheric air (AIR) is drawn in, via a filter 1, by a main air compressor 2 and is compressed to a pressure of approximately 15 bar. The compressed feed air 3 is cooled in a pre-cooling device 4. Said device can contain an aftercooler for indirect cooling or a direct contact cooler, or both. The pre-cooled feed air 5 is cleaned in a cleaning device 6 which is normally formed by a pair of switchable adsorbers. The compressed, pre-cooled and cleaned feed air 7 is cooled in a main heat exchanger 8 to approximately dew point and is introduced into the high-pressure column 10 via the line 9,

The high-pressure column 10 is part of the distillation column system which also has a low-pressure column 11, a main condenser 12 and a low-pressure-column top condenser 13. A first part 15 of the top gas 14 of the high-pressure column 10 is introduced into the liquefaction space of the main condenser 12, where it is at least partially condensed. Liquid nitrogen 16 formed in the liquefaction space of the main condenser 12 is introduced into the high-pressure column 10, where a first part serves as a return flow. A second part 17 is cooled in a counter-current subcooler 18 and is delivered (49) to the top of the low-pressure column 11.

A second part 19 of the top gas 14 of the high-pressure column 10 is guided, as a first compressed nitrogen product stream 19, via the line 20 to the main heat exchanger 8, where it is warmed to approximately ambient temperature. It is possible—as shown in FIG. 1—for the pressure of the warm compressed nitrogen 21 to be increased further, in principle to any desired discharge pressure, in a nitrogen compressor 22 with an aftercooler 23. Said compressed nitrogen is finally drawn off as compressed nitrogen product (PLAN). Should the desired product pressure not be higher than the high-pressure-column pressure (minus the pressure losses), the nitrogen compressor 22 and the aftercooler 23 can be omitted.

Liquid crude oxygen 24 is drawn off from the bottom of the high-pressure column 10, is cooled in the counter-current subcooler 18, and is fed to an intermediate point of the low-pressure column 11.

The top gas 26 of the low-pressure column 11 is introduced into the liquefaction. space of the low-pressure-column top condenser 13. The liquid nitrogen 27 formed there is introduced into the low-pressure column 11. The bottoms liquid 28 of the low-pressure column 11 is cooled in the counter-current subcooler 18 and is introduced via the line 29 into the evaporation space of the low-pressure-column top condenser 13 which is purged continuously or intermittently via a purge line 39. Gas formed there is warmed as tail gas 30 in the counter-current subcooler 18. The tail gas 31 downstream of the counter-current subcooler 18 is fed to the main heat exchanger 8 at the cold end, where it is warmed to an intermediate temperature. The tail gas 32 at the intermediate temperature is fed to a first tail-gas turbine 33, where it is expanded in a work-performing manner. The expanded tail gas 34 is reintroduced into the main heat exchanger 8 and is warmed as far as the warm end. The warmed tail gas 35 exits the plant at approximately ambient temperature. The tail-gas turbine 33 is mechanically coupled to the cold compressor 36 via a common shaft or a gear mechanism.

A nitrogen stream 37 is drawn off in gaseous form from the top of the low-pressure column 11, is compressed in the cold compressor 36 to approximately high-pressure-column pressure, is guided via a regulating valve 41, and finally is mixed, as a second compressed nitrogen product stream 38, with the first compressed nitrogen product stream 19 and warmed together therewith in the main heat exchanger 8 and finally drawn off as compressed nitrogen product (PGAN).

In order to cover the cold losses of the plant, the tail-gas turbine does not deliver its entire mechanical energy to the cold compressor 36, hut also drives a generator 40 which is seated on the same shaft or is connected to the same gear mechanism. Instead of the generator 40, a dissipative brake, for example an oil brake, can also be used.

In FIG. 2, use is made of two tail-gas turbines 33, 233, connected in parallel, one of which is coupled to the cold compressor 36 and the other of which is coupled to a generator 240 (or to a dissipative brake).

Whereas in FIGS. 1 and 2, the main air compressor 2 and the nitrogen compressor 22 are formed by two independent machines, in the exemplary embodiment of FIG. 3, use is made of a combined compressor 302 which fulfills both tasks. In the exemplary embodiment, said combined compressor has n=8 stages, of which i=5 stages form the main air compressor 2. The remaining n−i=3 stages form the nitrogen product compressor. This allows a PGAN final pressure from approximately 70 to 100 bar to be attained.

In the exemplary embodiment of FIG. 4, the two compressed nitrogen product streams 19, 38 are warmed in separate groups of passages of the main heat exchanger 8, The warmed nitrogen streams 419 and 438 are merged at 420. The second compressed nitrogen product stream 38 from the cold compressor 36 can thus be introduced into the main heat exchanger 8 at a higher temperature than the first compressed nitrogen product stream 19. Consequently, the process can be configured somewhat more favorably in terms of energy.

In FIG. 5, the energy transfer between the first tail-gas turbine 33 and the cold compressor 36 is realized, in contrast to FIG. 1, not mechanically, but electrically. For this purpose, the first tail-gas turbine 33 is mechanically coupled to an electrical generator 40. The electrical energy produced there is at least partially transferred via an electrical power grid to a motor 540, which is in turn mechanically coupled to the cold compressor 36 and drives this.

In comparison with FIG. 2, the entire tail gas in the generator-turbine 33/40 expands and thereby also produces the energy required for driving the cold compressor.

The specific features of FIGS. 2 to 5 can also be combined with one another as desired, for example to form a system having two tail-gas turbines and a combined compressor and two groups of passages in the main heat exchanger for the two compressed nitrogen streams. In all exemplary embodiments, the high-pressure column (with sieve trays) and low-pressure columns (with packings or sieve trays) are arranged one above the other. Alternatively, they can be installed next to one another. The invention is also suitable for offshore concepts, for example for floating installations for obtaining nitrogen for oil or gas fields (enhanced oil recovery—EOR).

FIG. 6 largely corresponds to FIG. 4, but here there is additionally a throttle valve 619 situated in the line 419. Here, a modification is shown with a product pressure of 10.9 bar, with a high-pressure-column pressure of 12.0 bar at the top. The nitrogen stream 37 from the top of the low-pressure column is compressed in the cold compressor 36 here accordingly only to 11.1 bar, i.e. not quite to the high-pressure-column pressure. Said stream is merged with the nitrogen stream 419, throttled. in the throttle valve 619, from the high-pressure column at 420 at the desired pressure of 10.9 bar.

In this case, it is important that the throttling 619 is performed downstream of the main heat exchanger 8, The throttling losses are thereby surprisingly greatly minimized and the pressure of the feed air can be reduced. The throttling from 12.0 bar to 10.9 bar can also be performed entirely or partially in the main heat exchanger 8, in that a correspondingly high pressure drop is selected there. As a result, the main heat exchanger 8 can be of especially compact structure.

FIG. 7 differs from FIG. 6 by virtue of the fact that the main condenser 12 and the low-pressure-column top condenser 13 used are not bath evaporators, but forced-flow evaporators. In said case, a purge stream 701 is drawn off from the bottom of the high-pressure column 10. A falling-film evaporator can alternatively be used as the main condenser 12,

In FIG. 8, the high-pressure column 10 and the low-pressure column 11 are arranged not one above the other, as in FIGS. 1 to 7, but next to one another. Otherwise, FIG. 8 does not differ from FIG. 4 or FIG. 5—depending on whether the nitrogen product is discharged at high-pressure-column pressure or at a slightly lower pressure (throttle valve 619). 

1. A method for obtaining a compressed nitrogen product by low-temperature fractionation of air in a distillation column system having a high-pressure column and a low-pressure column and also a main condenser and a low-pressure-column top condenser which are both designed as condenser-evaporators, wherein compressed, pre-cooled and cleaned feed air is cooled in a main heat exchanger and is at least in part introduced into the high-pressure column, top gas of the high-pressure column is introduced into the liquefaction space of the main condenser and at least part of the liquid nitrogen formed in the liquefaction space of the main condenser is introduced into the high-pressure column, top gas of the low-pressure column is introduced into the liquefaction space of the low-pressure-column top condenser and at least part of the liquid nitrogen formed in the liquefaction space of the low-pressure-column top condenser is introduced into the low-pressure column, bottoms liquid of the low-pressure column is introduced into the evaporation space of the low-pressure-column top condenser, gas formed in the evaporation space of the low-pressure-column top condenser is warmed as tail gas (in the main heat exchanger to an intermediate temperature, and at least a first part thereof is expanded in a work-performing manner in a first tail-gas turbine, reintroduced into the main heat exchanger, and warmed as far as the warm end of the main heat exchanger, a nitrogen stream is drawn off in gaseous form from the top of the low-pressure column, and a first compressed nitrogen product stream is drawn off in gaseous form from the top of the high-pressure column and is warmed in the main heat exchanger, characterized in that the mechanical energy produced in the first tail-gas turbine is at least in part used for driving a cold compressor, the nitrogen stream which has been drawn off in gaseous form from the top of the low-pressure column is compressed in the cold compressor to a pressure which is at least equal to the pressure of the first compressed nitrogen product stream, when the latter is drawn off from the high-pressure column minus 2 bar, and is subsequently warmed as a second compressed nitrogen product stream in the main heat exchanger, and the nitrogen stream which has been drawn off in gaseous form from the top of the low-pressure column is compressed in the cold compressor to a pressure which is at least equal to the pressure of the first compressed nitrogen product stream when the latter is drawn off from the high-pressure column minus 2 bar, and is subsequently warmed as a second compressed nitrogen product stream in the main heat exchanger, wherein the cold compressor overcomes a pressure differential which is at least equal to two thirds of the pressure differential between the top of the high-pressure column and the top of the low-pressure column.
 2. The method as claimed in claim 1, characterized in that the first compressed nitrogen product stream and the second compressed nitrogen product stream are mixed upstream of the main heat exchanger.
 3. The method as claimed in claim 1, characterized in that the first tail-gas turbine is mechanically coupled to the cold compressor via a common shaft or a gear mechanism.
 4. The method as claimed in claim 3, characterized in that the first tail-gas turbine is also mechanically coupled to an electrical generator or to an oil brake.
 5. The method as claimed in claim 1, characterized in that the first tail-gas turbine is mechanically coupled to an electrical generator, the cold compressor is driven by an electric motor, and the energy produced in the generator is at least partially electrically transferred to the motor.
 6. The method as claimed in claim 1, characterized in that a second part of the tail gas (warmed to the intermediate temperature is expanded in a work-performing manner in a second tail-gas turbine which is connected in parallel with the first tail-gas turbine.
 7. The method as claimed in claim 6, characterized in that the first tail-gas turbine is mechanically coupled to the cold compressor and the second tail-gas turbine is mechanically coupled to a generator or to a dissipative brake.
 8. The method as claimed in claim 1, characterized in that the first, the second or both compressed nitrogen streams are further compressed downstream of the main heat exchanger in a nitrogen compressor.
 9. The method as claimed in claim 8, characterized in that the feed air is compressed in a main air compressor which is formed by the first i stages of a combined n-stage compressor, where n≧2, i<n, and in that the nitrogen compressor is formed by the n−i last stages of the combined n-stage compressor.
 10. The method as claimed in claim 1, characterized in that the nitrogen stream which has been drawn off in gaseous form from the top of the low-pressure column is compressed in the cold compressor to a pressure which is at least equal to the pressure of the first compressed nitrogen product stream when the latter is drawn off from the high-pressure column.
 11. The method as claimed in claim 1, characterized in that the first compressed nitrogen product stream and the second compressed nitrogen product stream are warmed in separate passages and are in particular merged afterwards.
 12. The method as claimed claim 1, characterized in that at least one, more than one or all of the following measures are applied: design of the main condenser as a forced-flow evaporator, design of the main condenser as a falling-film evaporator, design of the low-pressure-column top condenser as a forced-flow evaporator.
 13. The method as claimed in claim 1, characterized in that the low-pressure column is arranged next to the high-pressure column, the main condenser is arranged above the high-pressure column, and the low-pressure-column top condenser is arranged above the low-pressure column.
 14. An apparatus for obtaining a compressed nitrogen product by low-temperature fractionation of air, comprising a distillation column system having a high-pressure column and a low-pressure column and also a main condenser and a low-pressure-column top condenser which are both designed as condenser-evaporators, comprising a main heat exchanger for cooling compressed, pre-cooled and cleaned feed air, wherein the liquefaction space of the main condenser is flow-connected to the top of the high-pressure column, the liquefaction space of the low-pressure-column top condenser is flow-connected to the top of the low-pressure column, and the evaporation space of the low-pressure-column top condenser is flow-connected to the bottom of the low-pressure column, and comprising means for extracting gas formed in the evaporation space of the low-pressure-column top condenser as tail gas, means for warming the tail gas in the main heat exchanger to an intermediate temperature, a first tail-gas turbine for expanding the partially-warmed tail gas in a work-performing manner, means for introducing the expanded tail gas into the main heat exchanger and means for extracting the warmed tail gas from the hot end of the warm heat exchanger, means for using mechanical energy, produced in the first tail-gas turbine, for driving a cold compressor, and a first compressed nitrogen product line for drawing off a gaseous first compressed nitrogen product stream from the top of the high-pressure column and for warming the first compressed nitrogen product stream in the main heat exchanger, characterized by means for drawing off a gaseous nitrogen stream from the top of the low-pressure column, means for introducing the gaseous nitrogen stream into the cold compressor, and means for introducing the cold-compressed nitrogen stream, as the second compressed nitrogen product stream, into the main heat exchanger, wherein the cold compressor is designed to overcome a pressure differential which is at least equal to two thirds of the pressure differential between the top of the high-pressure column and the top of the low-pressure column. 